It is usually required that enclosures for electronic components be shielded against penetration of electromagnetic radiation, since such radiation often can affect the operation of the components within. The individual panels making up these enclosures are metal and thus absorb or reflect this radiation. However, leakage can occur at the interface between the panels, and especially past removable panels which are necessary to provide access to the components for repair and maintenance. It has been known in the art to insure a seal at this interface by using a gasket comprising metal mesh strip having a metal mounting fin which may also have an elastomer core. The gasket is attached by its fin portion to the sheet metal surface with rivets, screws, structural adhesives or in some instances, solder or welds.
In addition, it is known to use such strip without a fin, bonded directly to an adjacent strip of elastomer, whereby the elastomer is used as a means of attaching the strip to a sheet metal surface using adhesives or screws. Both the bonded and fin type gaskets are typically used when equipment enclosures are made from heavy gauge sheet metal that will not bow, flex or corrugate when riveted or screwed. Thin gauge sheet metal will flex adjacent such rivets and screws causing gaps at the interface. This is known as "oil canning" and can result in radiation leakage. Reinforcement of the thin gauge sheets at the point of attachment of the rivets or screws prevents "oil canning". In the past, such reinforcement has been provided by metal strip material welded or riveted to the sheet where the mesh strip is to be attached.
Close proximity screw or bolt clamp spacing to prevent "oil canning" is not practical since most of the electronic modules require servicing at very short intervals and only quick-disconnect type fastening arrangements may be used. The latter dictate the amount of contact pressure available along every inch of the strip gasket interface. At least five pounds pressure per linear inch of strip gasket is necessary for effective shielding.
The problem of "oil canning" with thin gauge sheet metal (typically 0.020 inches to 0.060 inches) has also been solved by an elastomer core or plain metal mesh strip having a fin embedded in the split edge of an aluminum extrusion strip. The aluminum strip served no shielding function, but is used solely to attach the gasket to the metal sheet and provide structural support to the adjacent thin gauge sheet to prevent "oil canning."
However, there are several drawbacks to this mesh-aluminum extrusion gasket. The major drawback to aluminum extrusion is its cost. It is expensive to manufacture initially and to replace. Unfortunately, the split edge or jaws of the aluminum extrusion are clamped together on the mesh fin and eventually cut their way through it. Also, pivotal motion of the mesh strip about the clamped fin eventually causes fatigue and ultimately failure of the mesh adjacent the jaws.
In addition, the aluminum extrusion gasket cannot be readily cut to size, welded and drilled while being mounted on the equipment panel. Consequently, all aluminum extrusion type strip gasketing must be formed into a picture frame construction at the gasket supplier's plant using welding, cutting and drilling machines during various stages of manufacture. Then, the completed frame must be chromium coated for surface protection prior to final application of the coat of paint. Packaging of these large frames requires several sheets of cardboard plus heavy-wall cardboard boxes to protect the frame during transport from the supplier's facility. Even with this extensive degree of protection, a large percentage of the frames arrive damaged and unusable and must be replaced by the gasket supplier. A further disadvantage of the aluminum extrusion gasket is that maximum fabrication length is about fifteen feet.